Recently, as a demand for a high-speed and large-capacity service has increased, a mobile communication technology has developed to offer wireless data services of a high communication capacity, e.g., a 3rd generation partnership project (3GPP) long term evolution (LTE) mobile communication system. For this technology, there has been proposed a femtocell to provide communication services for a few subscribers located at home or an office.
While a macrocell is a service area covered by one base station in the existing cellular mobile communication, the femtocell covers much smaller area compared with that of the macrocell. In general, a femtocell base station which covers a femtocell may be a mini base station located indoors such as in home or an office, unlike a macro base station which covers a macrocell. Such a femtocell base station connects with the existing networks to not only provide inexpensive and various services with high-speed and large-capacity data communication but also achieve an expansion of a mobile communication service area, an improvement of a service quality and an increase of capacity of the mobile communication base station.
For example, a femtocell base station covering an area of about 10˜20 meter radius is located indoors to be connected to a wired IP network, thus providing wired/wireless communication through a mobile phone or various services associated with a home network. In addition, data can be transmitted from the femtocell base station to a switching device without a separate indoor relay, thereby reducing costs of establishing such a network.
Here, the femtocell base station can support a closed subscriber group (CSG) which allows connection only for an authorized user group. The femtocell covered by this femtocell base station is assigned a CSG identifier (ID) which is a unique ID and is called a CSG cell.
Furthermore, the femtocell base station can be operated in an open user access mode, a closed user access mode, and a mixed user access mode. In the open user access mode, a service may be allowed for all terminals regardless of their subscription to CSG and, in the closed user access mode, a service may be provided only for CSG subscribers. Further, in the mixed user access mode, CSG subscribers may be provided with a closed type service and CSG non-subscribers may be provided with in an open type service, thus offering differentiated services to each group.
Meanwhile, since users may install and operate femtocells for themselves, chances are that femtocell base stations can be established in an unplanned manner depending on the users' needs without considering a cell design by a service provider. Therefore, if a lot of femtocells are installed in a limited area, severe interference may be caused between the femtocell base stations. In addition, in case where a femtocell base station is operated in a closed user access mode, any non-subscriber terminals cannot receive services from the femtocell base station, and would request services to another femtocell base station or a macrocell base station which may be relatively far away. This may cause a considerable interference factor in the communication between the femtocell base station and other terminals.
As mentioned above, it is preferable to install a femtocell base station without causing any interference with adjacent base stations including a macro base station existing outdoors. Particularly, in case where a femtocell base station is operated in the closed user access mode, it is necessary to ensure a service quality of a subscriber terminal while minimizing interferences caused by a non-subscriber terminal.